1. Field of the Invention
This invention relates to an alkaline secondary battery provided with a paste-type electrode, and in particular to an alkaline secondary battery wherein a binder to be contained in the paste for the paste-type electrode is improved.
2. Description of the Related Art
The alkaline secondary battery is generally constructed such that a group of electrodes comprising a positive electrode and a negative electrode with a separator interposed therebetween, and an alkaline electrolyte are contained in place in a case. A paste-type electrode is utilized for these positive electrode and negative electrode. This paste-type electrode can be prepared for example by the steps of mixing an active material, a binder and water thereby forming a paste, filling the paste into a conductive substrate such as a perforated metal or mesh-like sintered fibrous substrate, and after being dried, molding the substrate.
Since the paste thus prepared is formed of an aqueous dispersion, a binder that has been conventionally employed is a water-soluble binder such as a water-soluble cellulose derivatives, typically methylcellulose (MC), carboxy methylcellulose (CMC) and hydroxypropyl methylcellulose (HPMC), polyvinyl alcohol (PVA), polyacrylic acid salts, polyacryl amide (PA), polyvinyl pyrrolidone (PVP) or polyethylene oxide (PEO).
However, if such an aqueous binder is employed together with an active material for example nickel hydroxide (Ni(OH)2) particles for preparing a paste-type positive electrode to be used for an alkaline secondary battery, the following problems will be raised. Namely, since this water-soluble binder is poor in flexibility due to its linear structure obtained from the polymerization of sole monomer, and also poor in bonding strength to the nickel hydroxide particles, the capability of this water-soluble binder for retaining the nickel hydroxide particles on the conductive substrate is naturally poor. Therefore, the secondary battery having such a paste-type positive electrode is accompanied with the problem that the nickel hydroxide particles are more likely to be fallen off from the positive electrode in the process of charge and discharge cycles. The nickel hydroxide particles thus fallen off may give rise to another problem that they contact with the negative electrode thereby causing an inner short circuit. Further, since the water-soluble binder is low in viscosity, it is hardly possible to stably disperse the nickel hydroxide particles in the paste. Moreover, since the water-soluble binder is poor in capability for absorbing an alkaline electrolyte, the amount of electrolyte to be held by the positive electrode will be decreased. As a result, the amount of redox reaction between the nickel hydroxide and the nickel oxyhydroxide (NiOOH) to be undergone in the presence of an alkaline electrolyte in the charge and discharge reaction will also be reduced. In particular, since the polyacrylic acid salt mentioned above is extremely poor in capability to absorb a salt-containing solution such as an alkaline electrolyte, the amount of the electrolyte retained by the positive electrode containing the polyacrylic acid salt becomes extremely low. Because of this, the amount of redox reaction is extremely decreased. Due to these problems as mentioned above, the conventional secondary battery is accompanied with problems that the internal pressure is increased in the process of charging/discharging cycles thereby shortening the cycle life of charging/discharging.
On the other hand, if such an aqueous binder is employed for preparing a paste-type negative electrode together with an active material for example hydrogen-absorbing alloy particles, and the negative electrode is used for the manufacture of an alkaline secondary battery, the following problems will be raised. Namely, since this binder swells when it absorbs an alkaline electrolyte, the swelling of the negative electrode is caused in the process of the charging/discharging cycles thereby causing the gas-absorbing capacity thereof to become gradually lowered. Further, the binder is poor in capacity to retain nickel hydroxide particles on a collector, and a paste containing such a binder is also poor in dispersion stability. Accordingly, in such a negative electrode comprising an active material having a larger specific gravity as compared with nickel hydroxide, for example hydrogen-absorbing alloy, the falling amount of the hydrogen-absorbing alloy during the process of charging/discharging cycles becomes more conspicuous, and the dispersion stability of the paste is extremely lowered. Due to these problems as mentioned above, the conventional secondary battery is accompanied with problems that the internal pressure is increased in the process of charging/discharging cycles thereby shortening the cycle life of charging/discharging.
If a paste is to be prepared from a mixture of homopolymers comprising polyvinyl alcohol and polyacrylate, some difficulty will be encountered in obtaining a homogeneous mixture of these polymer, making it very difficult to stably disperse the active material in the paste. Because even though the polyacrylic acid salt is soluble in water, the polyvinyl alcohol has a characteristics that when the molecular weight thereof is increased so as to be useful as a binder, the water-solubility thereof becomes decreased. If an alkaline secondary battery is provided with an electrode prepared from a paste which is poor in dispersion stability, the active material is caused to fall off in the process of the charging/discharging cycles, thereby causing an increase in internal pressure of the battery and shortening the cycle life of charging/discharging.